(i) Field of the Invention
The invention relates to soldering and tinning operations carried out using machines comprising a bath of liquid solder, wherein the bath is either a "static bath" or is set in motion as in the machines known as "wave soldering" machines.
These machines are used in particular for soldering electronic components on a support such as an electronic circuit, or for tinning the terminals of electronic components.
(ii) Description of the Related Art
Wave soldering machines are designed in such a manner that the circuits to be soldered (or the pieces to be tinned) are brought into contact with one or more waves of liquid solder generated by pumping a solder bath residing in a tank through a nozzle.
The pieces are generally fluxed beforehand in a zone upstream from the machine, primarily to deoxidize the metal surfaces in order to facilitate their subsequent wetting by the solder. The fluxing operation is followed by a preheating operation which is carried out both in order to activate the flux previously deposited on the circuit and in order to preheat the circuits and components prior to their arrival in the hot soldering zone.
The geometric configuration of the nozzle determines the shape of the solder wave. Wave soldering machines usually have two waves, a first so-called "turbulent" wave and a second so-called "laminar" wave that presents a relatively large flat upper surface.
In the absence of pieces to be soldered or tinned in the machine, the liquid solder in this laminar wave area flows at very low speed in the upstream direction of the machine. When a piece arrives in contact with the laminar wave, a partial reversal of the alloy flow occurs and part of the alloy flows in the downstream direction of the machine.
The machines are therefore usually provided with what can be described as a weir system, whose height can be used to control the flow rate of the downstream flow of the solder. This weir system can consist simply of a metal plate or a guide chute for returning the solder to the surrounding bath.
One notes that the flow rate and the direction of flow of the alloy in this laminar wave area have a determining influence on the quality of the resulting soldering.
It must also be noted that some users, in order to adapt to the very specific characteristics of their production, substantially limit this downstream solder flow, preferring to establish a very slight or nearly zero downstream overflow of solder.
Wave soldering (or tinning) machines are usually open to the ambient air atmosphere. One problem encountered by the users of such machines is the formation of oxide layers (called dross) at the surface of the solder bath as a result of its exposure to air, resulting in a not insubstantial loss of solder and the need to regularly clean the bath. For example, a medium-size machine can give rise to the formation of more than a kilogram of dross per hour of operation.
Considering the specific case of the laminar wave, it is readily understood that a zero or excessively small downstream overflow of solder will represent a major disadvantage since the dross constantly forming on the flat surface of the wave cannot be effectively eliminated and thus is deposited on the piece with significant adverse effects on the quality of the resulting soldering or tinning.
Without requiring any further description, it will be readily comprehended that this dross-formation phenomenon, described here at length for the case of the flat surface of the laminar wave in a wave soldering machine, applies even more to the flat surface of a static bath.
Various technical solutions have been heretofore proposed for protecting the solder bath from oxidation by the surrounding air. These solutions can be schematically divided into the following three categories.
a) A first category of solutions consists of setting up a confined protection atmosphere, at least above the solder bath, but also sometimes in the rest of the machine. Thus, completely inertized machines have appeared, designed from the outset as a gas-tight tunnel, but there have also appeared more simply cowling or hood systems that can be used on existing conventional machines open to the ambient air to set up a nitrogen blanket at least in the area of the solder bath.
In this first category of solutions, the applicant of U.S. Pat. No. 5,161,727 has proposed an inertization apparatus comprising a set of cowlings defining, at least directly above the solder bath, an interior space separated from the surrounding atmosphere by gas-tight means, with systems of gas injection channels opening into the upper parts of the diffuser-equipped cowlings.
While the inertization apparatus described in the aforesaid document certainly represents a substantial improvement over the performance of the existing systems (particularly in terms of optimizing the compromise between the flow rate of the injected gas and the concentration of residual oxygen achieved above the solder bath), this system still represents relatively complicated and costly designs since it must be virtually custom-fitted to each type of wave soldering machine present on the market.
b) A second category of solutions involves setting up an unconfined protective atmosphere using injectors localized in proximity to the solder wave without closure of the space above the waves.
The devices taught in WO 93/11653 fall into this second category.
Taking into account their extremely localized configuration, it is considered difficult to control the quality of inertization afforded by these processes, necessitating in practice the use of two symmetrical injectors to successfully achieve a low oxygen level.
Furthermore, none of the documents in this category of solutions deal with and provide solutions for the specific problems posed by laminar waves.
c) The third category of solutions to the problem of dross formation employs the use, at the surface of the laminar wave, of a film of oil with a high covering power.
The oil protection systems have the traditional disadvantages associated with the use of oil (particularly in the presence of a heat source), which in particular include the presence of oil deposits on the board (necessitating the implementation of an often difficult and imperfect cleaning), the necessity for scheduling frequent periods for machine maintenance due to the accumulation of oil in the solder bath, and oil vapor emissions which certainly represent a nuisance for the environment, whether for people or equipment.